In general, a TFT (Thin Film Transistor) drive circuit shown in FIG. 1 of the accompanying drawings is used as a drive circuit that drives each of pixels (each of light emitting cells) of a display panel when light emitting elements, such as organic electroluminescent materials (hereinafter, simply referred to as “organic ELs”), are arranged in a matrix fashion in the display panel. In FIG. 1, the cell is indicated by the broken-line square.
In FIG. 1, reference symbol Qa denotes a switching transistor for addressing, C denotes a memory capacitor that memorizes (holds) a voltage level of data, and Qb denotes a driving transistor that drives a load, i.e., organic EL light emitting element. Reference symbol EL denotes an organic EL light emitting element. The light emitting element EL has a structure in which an anode and a cathode sandwich an organic layer (or a layer which serves as an organic substance) that emits light. As depicted in the drawing, the light emitting element EL element has a rectifying property similar to that of a diode. In an actual display panel, the circuit shown in FIG. 1 constitutes each cell of the display screen, and a number of cells are arrayed in rows and columns (or X and Y directions) in a matrix on the screen.
The circuit of FIG. 1 operates as follows. First, a selection signal sent via an address line selects a desired cell out of a plurality of cells arranged in the display panel. The desired cell is a cell to emit. This selection signal turns the transistor Qa of the selected cell ON. Then the capacitor C of the same cell is electrically coupled with the data line, and the potential of the data line is memorized in the capacitor C. In other words, if the data on the data line is in an ON state, the data line potential is at a Hi level and the capacitor C is charged up to this Hi level potential. On the other hand, if the data is in an OFF state, its potential is at a Low level and the capacitor C is discharged down to the Low level potential.
Once the capacitor C has been charged up to the Hi level, the gate voltage of the transistor Qb is held at the Hi level until the Hi level data is replaced by a Low level data. Thus the transistor Qb continues to supply drain current to the load (i.e., organic EL light emitting element), and thereby the cell in which the Hi level data has been written continues to emit light. If a MOSFET (metal oxide semiconductor field effect transistor) is employed in the transistor Qb and such a drain-grounded circuit configuration shown in FIG. 1 is adopted, the input impedance of the transistor Qb will be substantially infinite. Then the potential of the capacitor C that has been charged does not decrease almost at all even if it is coupled with the transistor Qb.
In general, the electric property of low-temperature poly-silicon TFT, which is often employed in the driving circuit for organic EL light emitting elements, is subject to nonuniformities. Particularly, if the Vgs-Id (gate-source voltage-drain current) properties of the drive transistors Qb are not equal to each other among cells, the drive transistors also fluctuate in mutual conductance among the cells. In other words, even if the individual capacitors C in cells are charged by the same Hi voltage, different drain currents run in the drive transistors. Because of such nonuniformities in driving current for organic EL among the cells, a nonuniform brightness pattern may appear on the display screen. It is like strewing sand over the screen surface.